Superheater for chemical recovery unit



Nov. 15, 1960 y F. w. HocHMUTH 2,960,390

SUPERHEATER Foa CHEMICAL RECOVERY UNIT Filed July 3o, 1954 v 2 sheets-sheet 1 MNAYW/WMIHWWWWMMMM Illlllllllllllllllllllllllllllll Nov. ,15, 1960 F. w. HocHMUTH SUPERHEATER RCR CHEMICAL RECOVERY UNIT Filed July 30, 1954 2 Sheets-Sheet 2 FIG. 2.

FIG. 3.

mm M mm N W H N. IW K N A R F BY /z/ ATTORNEY SUPERHEATER FR CHEMICAL RECOVERY UNIT Frank W. Hochmuth, Scotch Plains, NJ., assignor to Combustion Engineering, Inc., New York, N.Y., a corporation of Delaware Filed July 30, 1954, Ser. No. 446,708

4 Claims. (Cl. 23-277) The invention relates to improvements in the arrangement of heat absorbing surfaces above the fuel burning portion of the combustion chamber of a steam generating unit burning a slag forming fuel and is particularly applicable to chemical recovery furnaces such as those in which chemical is recovered from the black liquor of wood pulp.

To burn black liquor having been heated to a predetermined temperature and raised to a predetermined density by evaporation the liquor is sprayed under pressure into the space in the furnace where the heat from the burning combustible constituents in the liquor is high enough to dry and partly volatilize the liquor. Part of the combustion occurs in this space but the greater part takes place on the hearth at the bottom of the furnace onto which the dried portions of the sprayed liquor fall as so called ash The combustion in the furnace occurs preferably in the presence of heat absorbing surfaces, such as water cooled tubes forming a part of a steam generator. The recovery units are self sustaining, that is, there is a commensurate amount of heat liberated in the furnace by the black liquor to compensate both for the heat given up to the absorbing surfaces and that required for the successful drying and burning of the combustible constituents on the hearth.

The products of combustion rising from the fuel bed on the hearth contain chemicals, mostly sublimated sodium salts, which yielding heat to the heat absorbing surfaces of the furnace condense into plastic and eventual solid states. The globules or particles of condensed chemicals occasion diiculties by adhering to and building up on the tubes of the heat absorbing surfaces which are located within the space of the furnace through which the products flow on their way to the boiler convection bank.

An object of the invention is to provide improved arrangements of heat absorbing surfaces over a furnace burning a slag forming fuel such as the chemical recovery furnace mentioned above whereby objectionable building up of condensed chemical upon the heat absorbing surfaces is substantially overcome.

Another object of the invention is to appreciably increase the heat absorbing capacity of the heating surfaces in the furnace without aggravating slagging diculties.

Additional objects and advantages will become apparent from the following description of an illustrative embodiment of the invention when read in conjunction with the accompanying drawings wherein:

Fig. 1 is a vertical longitudinal cross section through a pulp liquor recovery unit showing the improved arrangement of heat absorbing surfaces above the furnace.

Fig. 2 is a partial horizontal cross section taken on line 2-2 of Fig. l.

Figs. 3 and 4 are respectively partial horizontal and partial vertical cross sections taken on lines 3 3 and 4 4 of Fig. 1 showing the arrangement of spaced parnired States Patent O allelrows of heating surfaces which are staggered with j 2,960,390 Patented Nov. 15, 1960 respect to the general direction of llow of the products of combustion thereover.

In Fig. 1, A denotes a smelter furnace immediately above which is arranged a furnace chamber B. Adjacent to one side of the chamber B is arranged a boiler C. The chambers A and B have their walls lined with fluid cooled tubes 1 which connect into bottom headers 2 and into the top headers 3, thence into the drum 4, or said tubes connect directly into the drum 4 as shown. The boiler C comprises top and bottom drums 4 and 5 respectively which are interconnected by banks of tubes 6. A downcomer 7 connects bottom drum 5 to the bottom wall headers 2 via distributing tubes 8 and the top wall headers 3 connect via tubes 9 to top drum 4.

Black liquor evaporated to a desired density is projected into the furnace chamber-A through nozzles 10 at a desired pressure and temperature. A substantial portion of the sprayed particles gravitate to the furnace bottom in counter flow to the products of combustion rising from the smelting zone at the bottom of chamber A and are dried while thus falling. The suiciently dried particles collect on the hearth 11 forming a fuel bed. Preheated air at a desired temperature is delivered into the fuel bed through air nozzles 12 in amounts suicient to maintain a reducing atmosphere therein. The chemical ows out of the furnace in molten form from the hearth 11 through spout 13 and the combustibles in the products of combustion rising from the fuel bed are burned with preheated air delivered into the furnace above the fuel bed through additional air nozzles 14.

Combustion under normal operating conditions is usually completed near the bottom of the furnace chamber B. The fluid cooled furnace walls absorb heat from the rising products of combustion in suflicient amounts to lower their temperature, when leaving the top of the furnace chamber A, to approximately 1770o F. or at a temperature above the melting point of the chemicals entrained by the furnace gases.

Within furnace chamber B are parallel rows of heat absorbing tubes 15, 16, 17 and 18 the rows being spaced apart at a predetermined minimum distance and are aligned in the direction of ow of the products of combustion thereover. Rows 15 and 16 as well as rows 17 and 18 are staggered with respect to each other and are spaced apart a predetermined minimum distance in direction of gas flow. Each row of tubes 15, 16, 17, 18 comprises a multiplicity of aligned parallel tubes which substantially contact each other (Figs. 2 and 4). The rows 15 and i6-as shown in Fig. 1-each have their end tubes 19 and 20 connected into headers 21 and 22 respectively and the ends of their remaining intermediate tubes are serially interconnected by return bends 23 to form between headers 21 and 22 a continuous flow path for the cooling fluid through the tubes. Adjoining tubes of each row are fastened together as by welding at spaced intervals to maintain their alignment. The rows 17 and 18 are each similarly constructed as the rows 15 and 16 described above and have their end tubes 24 and 25 preferably connected to headers 26 `and 27 respectively to form between these headers a continuous serial flow path for the cooling fluid therethrough.

The tubes of rows 15 and 16 extend between the furnace wall cooling tubes 1 and are supported by the front and rear furnace walls. These tubes may in addition be supported intermediate their ends against side sway by fluid cooled pipes (not shown) arranged transversely to the rows and fastened thereto.

Each of 4the rows of tubes 15 and 16 forms in effect a continuous surface or plate of tubes exposed on opposite sides principally to the radiant heat of the hot i products of combustion flowing between these tube plates. 5

Because of the substantially continuous metallic surface presented by either side of the rows of tubes the slag which adheres to the tubes forms a continuous fluid surface flowing downwardly and dripping E the lower edge of tube plates and 16. This occurs aslong as the temperature of the furnace gases is sufficiently hot to maintain such flow. In places where the temperature of the furnace gases is insufhcient tol maintain a iluid ilow of slag, the slag may solidify and build up on the surface. However, due to the continuous and separate surfaces of slag on either side of a tube row and due to the relatively small portion of products of combustion contacting the row, objectionable slagging is minimized and any adhering slag is relatively easily removed by blowers. K

This relatively easy removal of the slag represents a great improvement in comparison with the removal of `the slag from the conventionally spaced apart tubes and more closely spaced rows of tubes used heretofore where the slag enveloped the individual tubes to form a continuous bond. In such designs the slag not only enveloped the tubes but built up and eventually spanned across rows unless frequently removed.

For normal self sustaining operating conditions in the furnace, the rows of tubes 15 and 16 are in a zone of heat which maintains a substantially fluid condition of the slag thereon. As the products of combustion enter the rows or tube panels 17 and 18 they have been cooled mostly by radiation of heat to the iiuid cooled furnace wa-ll tubes 1 and to the solid rows or panels 15 and 16 of fluid cooled tubes so that a temperature of approximately 1440io F. is reached where slag particles approach or in the main have reached a solid state. Slagging on the surface of tube panels 17 and 18 is accordingly substantially reduced. At said self sustaining operation the products of combustion leave the rows of tubes 18 and enter the boiler C at a temperature of about 1200 F. at which slag particles are in a substantially solid state.

Staggering each of the rows or panels of tubes 16 with respect to the preceding rows or panels of tubes 15 provides the advantage of permitting the slag to drip from the panels 16 without contacting the panels 15. Furthermore the produc-ts of combustion which flow between tube panels 15 have a relatively hot core iiowing midway therebetween, which hot core straddles the tube panels 16 as it leaves the tube panels 15 and is thereby broken up and intermixed with adjacent cooler products. Heat transfer to tube panels 16 is thereby substantially enhanced. The staggering of the widely spaced tube panels or rows accordingly permits an appreciable reduction of objectionable slagging and an eiective cooling of products o-f combustion rising from the furnace.

I have found that for successful reduction of slagging, the spacing of the tube panels 15 and 16 and the tube panels 17 and 18 should be at least about three times the diameter of the tubes of which these panels are made. The spacing 4in direction of gas iiow between the tube bank formed by tube panels or rows 15 and the -tube banks formed by tube panels or rows 16 should also be at least about three times the diameter of the tubes of which these panels are made. The same distance is preferably maintained between the tube banks formed by tube panels 17 and 18 respectively.

Such a spacing eliminates the bridging over of slag and provides an overall favorable increase in -furnace performance by more effective heat transfer, by substantially reducing slag accumulations and overcoming the necessity for too frequent slag removal during operation.

In the illustrative disclosure, the tube rows or panels 15 and 16 are cooled by boiler water subject to forced circulation by means, not shown, and the tube rows or panels 17 and 18 are part of a superheater for heating the steam collected in drum 4.

While a preferred embodiment of the invention has been here shown andA described, it will be understood that changes in construction, combination and arrangement of parts may be made without departing from the spirit and scope of the invention as claimed.

I claim:

1. In a vertical furnace fired with a `fuel in the lower portion thereof from which products of combustion entrain vaporized chemicals which condense and solidify upon cooling, a multiplicity of spaced panels forming a bank within the upper portion of the furnace, each panel comprising a multiplicity of substantially horizontal parallel contacting uid cooled tubes, each tube being in tangen-t relation with the adjacent tube, and vforming a wall lying in a vertical plane and extending across said upper portion, and at least one other bank of rows of panels within said upper portion constructed and arranged like said preceding bank and located thereabove, the rows of panels of each other subsequent bank being staggered with respect to the rows of the preceding bank.

2. In a boiler furnace fired with a yfuel in the lower portion thereof from which products of combustion entrain vaporized chemicals which condense and solidify upon cooling, a multiplicity of spaced panels forming a bank within the upper portion of the furnace, each panel comprising a multiplicity of substantially horizontal parallel contacting tubes, each tube being in tangent relation with the adjacent tube, and forming a wall lying in a vertical plane and extending across said upper portion, at least one other bank of spaced rows of panels formed of tubes within said upper portion constructed and arranged like said preceding bank and located therebeyond with respect to said products iiow, the panels of tubes of each subsequent bank being staggered with respect to the panels of the preceding bank and the spacing between said panels of tubes of each bank and between said banks being at least equal to about three times the diameter of said tubes, said panels of said banks being cooled by boiler water, and at least two banks of panel rows of superheater tubes located in said upper furnace portion above said banks of water cooled panels, each panel comprising a multiplicity of substantially vertical contacting tubes lying `in a vertical plane substantially aligned with the direction of the flow of said products of combustion thereover, the panels of tubes of adjacent banks of superheater tubes being staggered and the spacing between said panels of tubes of each superheater bank and between the adjacent superheater banks being at least equal to about three times the diameter of said tubes.

3. In a system for recovering chemical and generating steam from waste liquor of wood pulp mills having a smelter furnace into which the waste liquor is sprayed and from which products of combustion entrain vaporized chemicals which condense and solidify upon cooling, a chamber vertically above the furnace, steam generating tubes lining said furnace and said chamber walls, a multiplicity of spaced panels of steam generating tubes forming a bank within said charrrber, each panel comprislng a multiplicity of substantially horizontal parallel contacting steam generating tubes, each tube being in tangent relation with the adjacent tube, and forming a wall lying in a vertical plane and extending across said chamber, and at least one other bank of panels of tubes within said chamber constructed and arranged like the preceding bank and located thereabove, the panels of tubes of said subsequent bank being staggered with respect to the panels of said preceding bank and the spacing between said panels of tubes of each bank and between the adjacent panels of said banks being at least equal toY about three times the diameter of said tubes the amount of surface of said steam generating tubes on said furnace and chamber Walls and in said banks of tubes being sufficient to lower the temperature of said products of combustion beyond said banks at normal self sustaining rate of operation to about 1440 F.

4. In a system for. recovering chemical andV generatingsteam from. waste liquor ofv 'wood pulp mills havingY a smelter furnace into which the waste liquor is sprayed and from which products of combustion entrain vaporized chemicals which condense and solidify upon cooling, a chamber vertically above the furnace, closely spaced steam generating tubes lining said furnace and said chamber walls, a multiplicity of spaced panels of steam generating tubes forming a bank within said chamber, each panel comprising a multiplicity of substantially horizontal parallel contacting steam generating tubes, each tube being in tangent relation with the adjacent tube, and forming a wall lying in a vertical plane and extending across said chamber, at least one other bank of panels formed of tubes within said chamber constructed and arranged like said preceding bank and located thereabove, the panels of tubes of said subsequent bank being staggered with respect to the panels of the preceding bank and the spacing between said panels of each bank and between the adjacent panels of said banks being at least equal to about three times the diameter of said tubes the amount of surface of said steam generating tubes on said furnace and chamber walls and in said banks of tubes being sucient to lower the temperature of said products of combustion beyond said banks at normal self sustaining rate of operation to about 1440 F., and at least two banks of panels of superheater tubes located in said chamber above said banks of steam generating tubes, each panel comprising a multiplicity of substantially vertical contacting tubes lyng in a vertical plane substantially aligned with the direction of flow of said products of combustion through said chamber, the panels of tubes of adjacent banks of superheater tubes being staggered and the spacing between said panels of each bank and between the adjacent tubes of said banks being at least equal to about three times the diameter of said tubes, the amount of surface of said banks of superheater tubes being suicient to lower the temperature of said products of combustion beyond said banks at normal self sustaining rate of operation to about 1200-1250 F.

References Cited in the file 0f this patent UNITED STATES PATENTS 2,067,671 Kooistra Ian. 12, 1937 2,277,946 Badenhausen Mar. 3l, 1942 2,308,762 Krug et al. Jan. 19, 1943 2,416,462 Wilcoxson Feb. 25, 1947 2,594,267 Wilcoxson Apr. 22, 1952 FOREIGN PATENTS 630,857 Great Britain Oct. 24, 1949 

1. IN A VERTICAL FIRED WITH A FUEL IN THE LOWER PORTION THEREOF FROM WHICH PRODUCTS OF COMBUSTION ENTRAIN VAPORIZED CHEMICALS WHICH CONDENSE AND SOLIDIFY UPON COOLING, A MULTIPLICITY OF SPACED PANELS FORMING A BANK WITHIN THE UPPER PORTION OF THE FURNACE, EACH PANEL COMPRISING A MULTIPLICITY OF SUBSTANTIALLY HORIZONTAL PARALLEL CONTACTING FLUID COOLED TUBES, EACH TUBE BEING IN TANGENT RELATION WITH THE ADJACENT TUBE, AND FORMING A WALL LYING IN A VERTICAL PLANE AND EXTENDING ACROSS SAID UPPER PORTION, AND AT LEAST ONE OTHER BANK OF ROWS OF PANELS WITHIN SAID UPPER PORTION CONSTRUCTED AND ARRANGED LIKE SAID PRECEDING BANK AND LOCATED THEREABOVE, THE ROWS OF PANELS OF EACH OTHER SUBSEQUENT BANK BEING STAGGERED WITH RESPECT TO THE ROWS OF THE PRECEDING BANK. 